During recent years lightweight composite materials have been employed for structural panel members, particularly for high performance aircraft. Although composite materials such as those containing graphite have a very high strength/weight ratio, in lightweight applications it is often necessary to include stiffeners along the length of composite panels. The prior art includes a number of fabrication approaches wherein thin resin pre-impregnated composite sheets are deposited one on top of the other. However, at regularly spaced intervals, a mandrel is positioned on the sheets so that it separates a base layer from overlying layers deposited on the base layer. The multi-layered composite sheets are then exposed to high pressure and temperature, such as in an autoclave until the resin of the impregnating material is cured. The result is a panel with the integrally attached, regularly spaced stiffener sections where the mandrels exist. Conventionally, the mandrels may be of a removable type such as those employing a TEFLON material which leave hollow stiffener sections or the mandrels may be made of a lightweight material, such as foam, which remain as part of the structure.
The problem with TEFLON mandrels principally resides in the fact that they often become mechanically attached, i.e. locked on, to the composite sheets which makes it difficult for them to be removed or in the process may cause rupture of some of the composite sheets, thereby decreasing the integrity of the structure. In the instance where mandrels are retained within the structural member, the extra weight is a definite disadvantage.